Conventional magnetic recording requires a stationary recording head which makes contact with a moving surface of a magnetic medium, which may be in web, disk or drum form. Because of the tendency of such heads to abrade the magnetic medium, at least the surface layer of the magnetic medium generally is made from a durable material selected to resist abrasion. In recent years, photographic films have been developed which include on one side of a base material, various layers of photosensitive materials; and on the other side, a thin, virtually transparent layer of magnetic medium. The magnetic medium can be used, for example, during manufacture of the film to record information related to the film's characteristics; during use of the film in the camera to enable the camera to read and adjust itself in response to previously recorded information and/or to record information about camera settings and scene characteristics; and during processing or developing of the film to enable the processing equipment to read previously recorded information and adjust the processing parameters accordingly. However, because of the presence of the magnetic medium in a photographic film, the properties of the surface layer of the magnetic medium are chosen more for their photographic suitabilities than for their resistance to the sort of abrasion which can occur due to contact with a stationary recording head in a conventional recording process. Any abrasion on the film surface must be avoided since it would degrade the optical properties of the film and thereby prevent capture and reproduction of high quality images. As a result, if conventional recording apparatus were used to record to the magnetic recording medium on such a film, there would be a danger that the portion of the film containing the magnetic medium would be damaged due to abrasion. Use of a contact head is acceptable in a camera due to the low speed of recording and reading, but would not be acceptable in a high speed film manufacturing process. Moreover, even if a small amount of abrasion were acceptable in a manufacturing process, the abraded material would tend to build up on the recording head, thereby increasing the gap between the head and the recording medium and potentially degrading the signal applied to the recording medium to an unacceptable level. Also, such abraded material would tend to break loose from the head, resulting in loose dirt on the film which would degrade performance.
Another type of recording process, known as anhysteretic recording, has been used for many years for duplication of magnetic media, particularly video and audio tapes, without requiring the use of a recording head which may abrade the media. In this process, a master magnetic medium is prepared which typically has a magnetic coercivity about two to three times higher than the coercivity of the slave medium on which duplicate copies are to be made. The master medium may be provided in web or drum format and is prerecorded with the information to be duplicated. The master medium and the slave medium are then brought into intimate, stable contact, with no relative motion, in the presence of a decaying, alternating magnetic bias field; so that, a mirror image of the information on the master is transferred to the slave.
Since there is no abrasion between the master and slave media in conventional apparatus and processes useful for anhysteretic recording, the application of such apparatus and processes to record to photographic film having a magnetic medium would appear promising. However, recording to a photographic film introduces a number of problems not found in conventional duplicating of video and audio tapes. For example, applications for still camera photography will require recording of digital signals of relatively low bit density due to the characteristics of the magnetic medium on the film, likely no higher than 1000 bits per inch (39.4 bits per millimeter) which means that recording onto the film must be done with a very high degree of reliability since redundant recording will not be feasible. Much higher bit densities typically are used for data, video and audio applications, such as 10,000 bits per inch (394 bits per millimeter) for digital tape, 6,000 bits per inch (236 bits per millimeter) for floppy disks and 10,000 bits per inch (394 bits per millimeter) for Winchester disks. So that conventional, contact head equipment for reading the signals recorded to the photographic film will not damage the central image area of the film, it is desired to record only along the very edges of the film, rather than in the central portion of the magnetic medium as would be the case for non-photographic applications. Because the well known anticlastic effect will cause the edges of the slave medium to curl away slightly from an underlying master medium in drum format, intimate contact may be reduced in the very area where recording is desired. This effect is more pronounced with a photographic film than with a conventional magnetic tape for given levels of tension and elastic modulus, since the film is several times thicker than such tape. Moreover, air tends to become entrained between the slave and master media during recording at manufacturing web transport speeds, which causes loss of intimate contact and prevents good signal transfer from the master medium to the slave medium, particularly at higher velocities and with wider webs.
Pinch rollers have been used to squeeze air from between the media; but with a pressure sensitive photographic film, such rollers are undesirable since they may cause images on the exposed film to exhibit unsightly pressure marks. Various types of air clamping devices are also known which apply elevated pressure air to the media to force air from the interface and bring the media into the desired intimate contact. However, such devices tend to be very noisy, can actually cause instabilities by forcing air into the interface and are expensive to operate due to their continuous need for compressed air. Where the master medium and the slave medium are both in web format, recording onto a slave photographic film with the magnetic medium on the back of the film in some arrangements would require that the emulsion side of the film be in contact with a backing roll for the master and slave media, which may cause damage to the emulsion.
Thus, a need has existed for improvements to conventional apparatus and methods for anhysteretic recording to suit them for recording to photographic films bearing magnetic media. More particularly, a need has existed for anhysteretic recording apparatus and methods which can record along the edges or at the central portion of a photographic film, with low signal density and low loss of signal and which can efficiently remove air from the interface between the media being recorded at high speeds without risking pressure marking of or other damage to the film.